US researchers detect radioactivity from Fukushima in US air

The thing to realise about the presence of fission products in the environment is that they are not (in a general sense) naturally occurring isotopes. They are produced within nuclear reactors either by the fission process itself or by neutron activation within the reactor. Consequently, their detection means that the level will exceed the "normal" level by many orders of magnitude.

Researchers at the Pacific Northwest National Laboratory (PNNL) and the University of Texas at Austin have published a research paper describing the detection of radio-xenon in air sampled in the USA (Journal of Environmental Radioactivity, Volume 102, Issue 7, July 2011, Pages 681-68).

The pulse of radio-xenon (133Xe) was detected in the week following the Japanese earthquake and tsunami which crippled the Fukushima nuclear power plant. The researchers estimated that the signal was 10000 to 100000 times above the normal level seen in US air.

Researchers and health physicists stressed that the increased quantity of radio-xenon over the USA was not a health concern. According to Tracy Tipping, a health physicist and laboratory manager at The University of Texas at Austin's Nuclear Engineering Teaching Laboratory, the typical exposure to radiation in the USA is 16.4 microsieverts per day. The additional component from the Fukushima accident pushed this figure up to 16.4017 microsieverts, a negligible increase (the maximum permissible exposure level for a radiation worker is 20 milisieverts per year).

Nevertheless, the findings provide a valuable insight into the Fukushima accident and highlight the global transportation of material from such a nuclear incident. The data also illustrates the use of nuclear forensic techniques for monitoring both declared and illicit nuclear activities around the world. One of the reports authors, Cockrell School of Engineering Associate Professor Steven Biegalski of Austin University, was on secondment at PNNL in Richmond, Washington (some 7000 km from Fukushima) and was involved a project to improve the sensitivity of the equipment used to detect the radioisotope.

"I think the conclusion was that this was a really major event here," Biegalski said of the Fukushima disaster. "The culmination of international research collaborations resulted in this very sensitive monitoring technology. These advancements will not only be beneficial for nuclear monitoring, they are also very beneficial to the emergency response teams called to disasters like Fukushima," he added. Biegalski is an expert in nuclear forensics, nuclear modelling, and nuclear monitoring.

The atmospheric release of radio-xenon was more significant in the Fukushima accident than in the Chernobyl or Three Mile Island accidents since three reactors lost their cooling systems and vented during the Fukushima accident.

133Xe is radioactive analogue of the noble gas xenon. It is used in medical science to study blood flow through the brain and the passage of air through the lungs. 133Xe is predominantly a beta emitting radionuclide that has a half-life of 5.2 days (this means that all of the radio-xenon released by the accident has now decayed completely since the accident occurred more than 150 days ago - it would have disappeared after 52 days).

133Xe is a fission product from the decay of 235U which takes place within a nuclear reactor (or weapon). Under normal circumstances, it would not be released to the environment so the detection of this radionuclide is of considerable importance in ensuring that nations are in compliance with their obligations under the Nuclear Non-Proliferation Treaty (if they are signatories of it) and the Comprehensive Nuclear test Ban Treaty.

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